The present disclosure relates to quantum computing, and more particularly to a quantum circuit for shifting the phase of a target qubit based on a control qubit.
By using the principle of quantum mechanics, a quantum computer performs calculation based on the qubits represented by the superposition of ‘0’ and ‘1’, so that it may have a much higher calculation speed than a digital computer using bits represented by only ‘0’ or ‘1’. With the development of quantum computing technology, any arbitrary single-qubit gate or Z-rotation gate may be implemented with relatively high accuracy. However, as the amount of computation increases, it is difficult to perform scalable quantum computing in a currently-implemented gate having a small error rate. To solve this, calculation of a fault-tolerant scheme which guarantees an arbitrary probability of success is required. In order to ensure the reliability of quantum computing, the operation of all quantum algorithms should be expressed by a universal set proposed from the fault tolerant protocol.
It is difficult to make all calculation devices required in any quantum algorithm in advance. Therefore, if a quantum algorithm to be executed is given, a process, in which a quantum algorithm is represented by a finite number of basic gates, is required. This process may be referred to as circuit synthesis problem, and the basic gates of quantum computing may be implemented with a universal set including Hadamard gate (H-gate), Phase gate (S-gate), π/8 phase shift gate (T-gate), and Controlled-NOT (CNOT) gate. Although the quantum algorithm has lower computational complexity than the conventional algorithm in the problem of factoring, when the quantum algorithm is decomposed into the basic gates described above, due to the increase in the number of gates, the advantages described above may be offset. Thus, there is a need to reduce the number of such gates.
Furthermore, there is a need for circuit synthesis for 2-qubit gates. Among the 2-qubit gates, a Controlled-Rn gate, which shifts the phase of the target qubits based on the control qubits, receives attention in that it constitutes a key part of many other quantum computations such as Quantum Fourier Transform (QFT). Thus, the decomposition of the Controlled-Rn gate is important in the overall part of the quantum algorithm, and there is a need to reduce the number of gates during the decomposition of the Controlled-Rn gate.